Leucine Aminopeptidase LAP Test Guide
An original applicator-tip visual for reading LAP disk reactions after cinnamaldehyde reagent is added.
Bench card visual
LAP reactions A-B
Read first
Positive develops red color quickly after reagent; negative stays unchanged or slightly yellow.
A
PositiveRed color on disk/tipLAP activity releases beta-naphthylamine, which forms red color after cinnamaldehyde reagent.
B
NegativeNo color or slight yellowNo red endpoint develops during the expected read window.
PositiveRed color on disk/tip
LAP activity releases beta-naphthylamine, which forms red color after cinnamaldehyde reagent.
NegativeNo color or slight yellow
No red endpoint develops during the expected read window.
Timed endpoint anchors
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Red color | Cinnamaldehyde detected beta-naphthylamine released by LAP | Positive LAP |
| No color change | No detectable LAP reaction | Negative LAP |
| Slight yellow | Background reagent color without red endpoint | Read as negative |
| Delayed red | Color outside the read window may be unreliable | Use validated timing |
LAP test study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Positive | Red color develops after cinnamaldehyde | Supports LAP-positive catalase-negative Gram-positive cocci such as Enterococcus in the correct workup. |
| Negative | No red color or only slight yellow | Supports LAP-negative branches such as Aerococcus/Leuconostoc-style separation in the right context. |
| Invalid or weak | Poor disk condition, flooding, or weak inoculum | Repeat with a fresh disk and proper colony load. |
Key takeaways
- LAP is a rapid enzyme disk test.
- The key visual endpoint is red color after reagent.
- Use LAP with catalase, PYR, bile esculin, vancomycin behavior, and organism morphology.
Remember
Best mental model: LAP frees the target compound; cinnamaldehyde turns that product red.